The invention is based on a device for measuring at least one parameter of a medium flowing in a line, and particularly for measuring a parameter of aspirated air in an internal combustion engine.
A device is already known (German Patent Disclosure DE 197 35 664 A1) in which a measuring element for measuring a parameter of air flowing in a line is disposed inside a tubular body through which a medium (air) flows. The upstream end of the tubular body extends as far as the inside of a filter chamber, where it has insertion openings on the jacket face, to prevent the imposition of dirt particles or water droplets on the measuring element. Especially if the air is quite dirty and there is a high proportion of water in the aspirated air of the engine, the danger exists that the air filter will become saturated with water, which then passes through the filter mat and will thus entrain dirt particles. On the downstream side of the air filter which is the actual clean side, there is then the danger that the aspirated air will again entrain dirt particles and water droplets from the filter surface that are then deposited undesirably on the measuring element and cause incorrect measurements or even failure of the measuring element. The tubular body in the prior art, by the disposition of the insertion openings on jacket face, lessens the risk of deposits on the measuring element, but this embodiment causes an undesired pressure drop that leads to a loss of measurement sensitivity.
From German Patent DE 44 07 209 C2, a measurement body for measuring the mass of aspirated air is known that can be inserted into the clean conduit of the intake line of an internal combustion engine and has a flow conduit which is composed essentially of a measurement conduit that narrows in the flow direction and an S-shaped deflection conduit adjoining it. The measuring element is disposed in the narrowing measurement conduit. The measuring element can, as is known from German Patent Disclosure DE 43 38 891 A1, for instance, be embodied as a micromechanical sensor part with a dielectric diaphragm.
The device of the invention has the advantage over the prior art that in a simple way, the imposition of dirt particles and liquid on the measuring element is prevented, without causing disadvantageous pressure losses. While the dirt particles and liquid droplets entrained by the medium, such as the aspirated air of an internal combustion engine, are trapped by the protective screen and diverted into a region of the air flow, or the inner conduit wall, that does not strike the measuring element, the flowing aspirated air passes virtually unhindered through the protective screen to reach the measuring element.
One possible way of diverting dirt particles and liquid droplets in a desired direction is advantageously obtained if the protective screen extends in inclined fashion in the flow direction.
It is advantageous, to provide an open outflow opening between a downstream end of the protective screen and an inner conduit wall of the flow conduit, as a result of which the liquid caught by the protective screen, with any dirt particles the liquid may contain, reaches a wall region of the tubular body, where it is entrained downstream by the flowing air while still adhering to the wall.
It is also advantageous that the measuring element is disposed in a measurement body that protrudes into the flow conduit, and a protective screen is located entirely or only partly upstream of the measurement body, so that liquid droplets and dirt particles will be reliably caught by the protective screen and detoured or diverted into the peripheral region of the tubular body. It is also advantageous if the measuring element is disposed in a measurement body, which extends along a longitudinal axis and which protrudes through an insertion opening in a first wall portion of the tubular body into the flow conduit in the direction of a second wall portion of the tubular body, and the protective screen defines a screen face which forms an angle of  less than 90xc2x0 with the longitudinal axis and is inclined in the direction of the second wall portion, so that the liquid droplets and dirt particles diverted from the protective screen are made to bypass the measurement body, below or beside it.
In a further advantageous feature, the measuring element is disposed in a measurement body, which extends along a longitudinal axis and which protrudes through an insertion opening in a first wall portion of the tubular body into the flow conduit in the direction of a second wall portion of the tubular body, and the protective screen defines a screen face, which extends approximately parallel to the longitudinal axis, so that liquid droplets and dirt particles diverted from the protective screen are made to bypass the measurement body laterally.
To assure the most homogeneous possible flow at the measuring element, it is advantageous to dispose a flow rectifier in the line, upstream and/or downstream of the tubular body.
To make the flow more uniform, advantageously, between an inner wall of the line and the tubular body, at least two flat struts are provided, oriented in the flow direction and transversely to the flow direction.
It is also advantageous to provide a suction extraction opening, upstream near a downstream end of the protective screen, leading to the line in the wall of the tubular body, by means of which line the liquid droplets and dirt particles diverted from the protective screen can be carried immediately out of the tubular body.